Such sensors are used as so-called lambda sensors, for instance, to determine the oxygen concentration in the exhaust gas of an internal combustion engine. The sensor is equipped with its own electric heating device so as to bring the sensor element to its operating temperature as quickly as possible, i.e., already during the warm-up phase of the engine. The sensor element is made of ceramic materials. However, such ceramics are very sensitive to great temperature fluctuations, which lead to tears in the ceramics and thereby to malfunctions and even a total breakdown of the sensor. Extreme temperature fluctuations, also known as thermo shock, occur, for instance, during the start and in the warm-up phase of the internal combustion engine if a cold water droplet strikes the already heated sensor element. Such water droplets may form in that the water vapor produced by the engine combustion condensates on cold surfaces of the exhaust system and of the sensor during the warm-up phase, and water droplets from the condensate are entrained by the gas stream and then reach the sensor element.
In a known sensor, which is used as exhaust sensor (German Patent No. 199 24 319), a double sheath tube, which is made up of an inner and an outer tube each of which is provided with gas-entry and exit openings, protects the sensor element from the action of water droplets entrained by the exhaust flow. A flow element for the exhaust gas is positioned at least one entry opening of the inner tube and/or at least one entry opening of the outer tube, the flow element diverting the exhaust gas entering the interspace enclosed by the inner and outer tube and/or entering the inner space of the inner tube, in the direction of the individual inner surface area of the inner and/or outer tube. This retains the water on the inner surface areas, and the water eventually evaporates as a result of the temperature of the exhaust gas, which increases as the internal combustion engines heats up.